1. Field of the Invention
The present invention relates to a spark plug used for ignition of an internal combustion engine.
2. Description of the Related Art
As for spark plugs used for ignition of an internal combustion engine such as an automobile engine, many spark plugs of the type having a first precious metal tip bonded to the vicinity of a leading end of a ground electrode body (i.e., the end opposite the end of the ground electrode body that is bonded to a metal shell) have been proposed for the following reason. This type of spark plug is provided so that an igniter portion of the spark plug protrudes into a combustion chamber to improve ignitability of the spark plug. Accordingly, the igniter portion is exposed to a high temperature.
While the demand for higher engine performance has recently increased, greater improvement in ignitability has been required of the spark plug. Increasing the axial-direction distance (hereinafter also referred to as protrusion amount) between a leading end surface of a first precious metal tip (bonded to the ground electrode body) opposite a center electrode and an inner circumferential surface of the ground electrode is effective for improving ignitability. This is for the following reason. A flame core generated in a spark discharge gap formed between the center electrode and the ground electrode (or the first precious metal tip) will tend to grow due to swirling or the like. However, there is a possibility that the flame core will contact the ground electrode body so as to impede its growth (hereinafter also referred to as extinction action) if the protrusion amount is small. Therefore, a structure in which the protrusion amount of the first precious metal tip bonded to the ground electrode body is made large to thereby accelerate growth of the flame core has been widely used (see Japanese Patent Laid-Open No. 2001-345162).
However, even in a spark plug having an increased protrusion amount as described in Japanese Patent Laid-Open No. 2001-345162, the extinction action may still occur to impede growth of the flame core. This is because the spark plug has a structure in which the flame core contacts the ground electrode body or the insulator while it is growing. For this reason, ignitability cannot be satisfactorily ensured.